If the atheist's intent is to convert someone away from their faith, religion, or belief and s/he presents their argument as definitive fact, then yes, it would be considered proselytizing.
. . . I often get into some pretty intense theological and scientific arguments in the brick and mortar world. One of the things I see that in my opinion is an utter and complete distortion of a healthy debate is when one or both sides of the debate start to make judgments concerning motive, or other internal states of mind, that are not really open to objective analysis through the dialogue taking place.
If a person believes strongly there's no God, and is willing to prove it in a well-thought out manner, using powerful argumentation, then naturally their strong belief, and willingness to back it up, can be interpreted as proselytizing even if their true motive is less to force anyone to believe what they say, and more simply to prove the veracity of what they say.
It's a fine line between proving the veracity of your belief versus trying to "convert" someone to that belief since argumentation always has some of the latter as a necessary ingredient of belief and argumentation. And it's a slippery slope, filled with moral hazard, rotting cabbage, and black-eyed peas, to deify either side of a debate as able to use 3D glasses to look into the secret motives of the other side of the debate. No person, imo, possesses the 3D glasses that can peer into motive.
John